If you can find the diaphram, I'd be surprised if the diaphram alone isn't several hundred dollars. If you can't repair the diaphram/wastegate, then you're looking at spending even more. While you can replace the portion of the manifold (which is prone to cracking even if you have a good wastegate) that holds the wastegate, that piece will set you back a few hundred bucks (not sure exactly...my car is an 89 with an internal wastegate, but I have a friend who had to do it...which brings me to the next point). Then you'd need to either decide how to integrate an alternate aftermarket external wastegate or replace the turbo with one of an internal wastegate design.
If the car is good otherwise, it might still be worth pursuing, but I'd be prepared for a real hastle to sort it out, and I'd budget at least $1,000. It could be less if you can find a diaphram, but I doubt it and it could easily be more, depending on the path you take.